TL;DR: Competitors in integrating with Atlassian are not allowed to incorporate code after the change because they used it in free add-ons, which caused the official integration (a paid add-on that is the sole source of funding) to be labeled a scam by a review in late August.

Plus, the thing was never really open source anyway:

draw.io is also closed to contributions, as it's not open source. We follow a development process compliant with our SOC 2 Type II process. We do not have a mechanism where we can accept contributions from non-staff members.